Global demand for liquid transportation fuel is projected to strain the ability to meet certain environmentally driven goals, for example, the conservation of oil reserves and limitation of greenhouse gas emissions. Such demand has driven the development of technology which allows utilization of renewable resources to mitigate the depletion of oil reserves and to minimize greenhouse gas emissions.
Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a food grade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase in the future.
Methods for the chemical synthesis of isobutanol, an isomer of butanol, are known, such as oxo synthesis, catalytic hydrogenation of carbon monoxide (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719) and Guerbet condensation of methanol with n-propanol (Carlini et al., J. Molec. Catal. A: Chem. 220:215-220, 2004). These processes use starting materials derived from petrochemicals, are generally expensive, and are not environmentally friendly. The production of isobutanol from plant-derived raw materials would minimize greenhouse gas emissions and would represent an advance in the art.
2-Butanone, also referred to as methyl ethyl ketone (MEK), is a widely used solvent and is the most important commercially produced ketone, after acetone. It is used as a solvent for paints, resins, and adhesives, as well as a selective extractant, activator of oxidative reactions, and it can be chemically converted to 2-butanol by reacting with hydrogen in the presence of a catalyst (Nystrom, R. F. and Brown, W. G. (J. Am. Chem. Soc. (1947) 69:1198). 2,3-butanediol can be used in the chemical synthesis of butene and butadiene, important industrial chemicals currently obtained from cracked petroleum, and esters of 2,3-butanediol can be used as plasticizers (Voloch et al., “Fermentation Derived 2,3-Butanediol,” in Comprehensive Biotechnology, Pergamon Press Ltd., England, Vol. 2, Section 3:933-947 (1986)).
Microorganisms can be engineered for the expression of biosynthetic pathways that utilize pyruvate to produce, for example, 2,3-butanediol, 2-butanone, 2-butanol and isobutanol. U.S. Patent Application Publication No. US 2007/0092957 A1 discloses the engineering of recombinant microorganisms for production of isobutanol. U.S. Patent Application Publication Nos. US 2007/0259410 A1 and US 2007/0292927 A1 disclose the engineering of recombinant microorganisms for production of 2-butanone or 2-butanol. Multiple pathways are disclosed for biosynthesis of isobutanol and 2-butanol, all of which initiate with cellular pyruvate. Butanediol is an intermediate in the 2-butanol pathway disclosed in U.S. Patent Application Publication No. US 2007/0292927 A1.
Engineering recombinant host cells for increased availability of pyruvate and/or for reduced glucose repression allows for increased formation of the products of pyruvate-utilizing biosynthetic pathways. For example, reducing glucose repression has been used to improve the respiratory capacity of yeast and to increase biomass production. Also, International Publication No. WO 1998/26079 A1 discloses overexpression of the Hap1 transcription factor to reduce glucose repression results in increased respiratory capacity and increased biomass production. European Patent No. 1728854 discloses a process for biomass production using yeast overexpressing the Hap1 transcription factor grown in aerobic conditions.
Functional deletion of the hexokinase 2 gene has been used to reduce glucose repression and to increase the availability of pyruvate for utilization in biosynthetic pathways. For example, International Publication No. WO 2000/061722 A1 discloses the production of yeast biomass by aerobically growing yeast having one or more functionally deleted hexokinase 2 genes or analogs. In addition, Rossell et al. (Yeast Research 8:155-164 (2008)) found that Saccharomyces cerevisiae with a deletion of the hexokinase 2 gene showed 75% reduction in fermentative capacity, defined as the specific rate of carbon dioxide production under sugar-excess and anaerobic conditions. After starvation, the fermentation capacity was similar to that of a strain without the hexokinase 2 gene deletion. Diderich et al. (Applied and Environmental Microbiology 67:1587-1593 (2001)) found that S. cerevisiae with a deletion of the hexokinase 2 gene had lower pyruvate decarboxylase activity.
Functional deletion of the pyruvate decarboxylase gene has also been used to increase the availability of pyruvate for utilization in biosynthetic pathways. For example, U.S. Application Publication No. US 2007/0031950 A1 discloses a yeast strain with a disruption of one or more pyruvate decarboxylase genes and expression of a D-lactate dehydrogenase gene, which is used for production of D-lactic acid. U.S. Application Publication No. US 2005/0059136 A1 discloses glucose tolerant two carbon source independent (GCSI) yeast strains with no pyruvate decarboxylase activity, which may have an exogenous lactate dehydrogenase gene. Nevoigt and Stahl (Yeast 12:1331-1337 (1996)) describe the impact of reduced pyruvate decarboxylase and increased NAD-dependent glycerol-3-phosphate dehydrogenase in Saccharomyces cerevisiae on glycerol yield. U.S. patent application Ser. No. 12/477,942 discloses increased conversion of pyruvate to acetolactate by engineering yeast for expression of a cytosol-localized acetolactate synthase and substantial elimination of pyruvate decarboxylase activity.
There remains a need to improve redox balance, glucose consumption and/or product formation of a pyruvate-utilizing biosynthetic pathway in recombinant host cells comprising a functional deletion of genes encoding dual-role hexokinases such as the hexokinase 2 gene.